9 Little-Known Side Effects of Too Much Fish Oil

Fish oil is widely recognized for its array of health-supporting benefits. It supplies two primary omega-3 fatty acids:

• eicosapentaenoic acid (EPA)
• docosahexaenoic acid (DHA)

Packed with these heart-friendly omega-3s, fish oil supplements may help reduce inflammation, lower diabetes risk, and ease symptoms of inflammatory disorders such as rheumatoid arthritis.

That said, more fish oil isn’t always better — taking excessively large amounts can potentially cause adverse effects.

This article outlines nine possible side effects linked to consuming too much fish oil or omega-3 supplements.

Low blood pressure

Daily fish oil intake can help decrease elevated blood pressure.

A suggests that doses between 2 and 3 grams (g) of omega-3 fatty acids may reduce both systolic and diastolic blood pressure:

• 2 g of omega-3 fatty acids: lowered systolic blood pressure by 2.61 and diastolic blood pressure by 1.64
• 3 g of omega-3 fatty acids: lowered systolic blood pressure by 2.61 and diastolic blood pressure by 1.8
• More than 3 g of omega-3 fatty acids: may have a greater effect

If you already have low blood pressure or are taking medications to manage it, using fish oil supplements could push your blood pressure too low.

Discuss with your healthcare provider before starting fish oil or any new supplement, particularly if you have existing medical conditions.

Acid reflux

Some individuals report acid reflux (heartburn) and other gastrointestinal complaints after taking fish oil, possibly due to its high fat content.

2019 research examining fish oil side effects found that acid reflux and other digestive issues were uncommon.

Acid reflux symptoms can include:

• heartburn
• belching
• nausea
• stomach discomfort
• bloating

When and how you take fish oil may influence these symptoms. Using a moderate amount with meals may lessen reflux and ease discomfort. Splitting the total dose into smaller portions throughout the day can also reduce indigestion.

Diarrhea

Diarrhea is a common complaint linked to fish oil usage and may occur more often with high intakes.

The same 2019 study noted reports of diarrhea and other digestive adverse effects with larger fish oil doses that provided 3 to 4 g (3,000 to 4,000 milligrams) of EPA and DHA.

Other omega-3 formulations can also produce diarrhea.

If diarrhea occurs after taking omega-3s, try taking supplements with food and consider reducing the dose to see if symptoms improve.

Headache

The National Center for Complementary and Integrative Health (NCCIH) lists headaches among possible side effects of omega-3 supplements like fish oil.

Conversely, some studies indicate that fish oil may lessen the severity of migraine attacks in people with migraine, a neurological disorder characterized by intense headaches and other symptoms.

Fishy odor

Trimethylamine is a compound present in fish oil.

Although rare, some people cannot metabolize this compound effectively. This condition is known as trimethylaminuria, or fish odor syndrome.

If you cannot break down trimethylamine, it can accumulate and be released via:

• breath
• sweat
• urine

Taking fish oil when you have this condition may cause bodily fluids to emit a fishy smell, which can fluctuate over time.

Atrial fibrillation

Among people with cardiovascular disease or those at elevated cardiovascular risk, taking very high amounts of fish oil for years may slightly raise the chance of atrial fibrillation (AFib).

AFib is an irregular heart rhythm that disrupts the normal flow of blood through the heart. Having AFib increases the risk of blood clots and stroke.

A large 2020 clinical trial observed this potential risk in participants who took 4,000 milligrams (mg) of fish oil per day, which is far above the content of a single typical supplement.

Impaired immune response

One benefit of fish oil is its anti-inflammatory effect, achieved by dampening inflammatory pathways.

However, inflammation also plays a role in fighting infections, healing injuries, and handling diseases.

Suppressing inflammation with high daily doses of fish oil (900 mg EPA plus 600 mg DHA) for several weeks or more may diminish immune function.

Increased bleeding

In theory, fish oil could increase bleeding by slowing clot formation. Yet a indicates that fish oil supplements did not raise bleeding during surgeries and actually reduced the need for blood transfusions.

A of clinical trials also suggests omega-3s are not linked to higher bleeding risk. The authors note that very high-dose purified EPA could theoretically raise bleeding risk, but this was not significant in practice.

Some clinicians may still advise stopping fish oil prior to surgery. Consult your doctor before taking fish oil if you use blood thinners like warfarin (Jantoven).

Risk of vitamin A excess

Vitamin A is stored in the body, so excess intake can accumulate over time.

Certain omega-3 supplements are rich in vitamin A, which can become toxic at high intakes. For instance, one tablespoon (14 g) of cod liver oil can supply a large portion of your daily vitamin A requirement in a single serving.

Vitamin A toxicity can cause side effects such as:

• dizziness and coordination difficulties
• nausea
• joint discomfort
• skin irritation
• severe headache

Prolonged excess intake may also lead to liver damage and, in extreme cases, liver failure.

Monitor the vitamin A content of your omega-3 product and keep doses moderate.

The recommended dietary allowance of vitamin A for adults over 18 is:

• males: 900 micrograms (mcg) retinol activity equivalents (RAE)
• females: 700 mcg RAE
• pregnant people: 770 mcg RAE
• breastfeeding people: 1,300 mcg RAE

RAE units account for differing bioactivities of retinol and provitamin A carotenoids that the body converts to retinol.

How much is too much?

The and the advise consuming about 8 ounces or roughly of fatty fish per week.

A standard supplement often contains about 1,000 mg of fish oil, providing roughly 180 mg EPA and 120 mg DHA — about 300 mg combined. One teaspoon (5 milliliters) of liquid fish oil typically has around 1,300 mg.

Adequate Intake values, which vary by individual factors, are:

• Males 14 and older: 1.6 g (1,600 mg)
• Females 14 and older: 1.1 g (1,100 mg)
• Pregnant people: 1.4 g (1,400 mg)
• Lactating people: 1.3 g (1,300 mg)

However, have administered higher doses — up to 4 g (4,000 mg) — for certain conditions like heart disease or elevated triglycerides.

The European Food Safety Authority indicates omega-3 supplements are safe at doses up to daily.

As a practical guideline, if you notice adverse effects, lower your intake or aim to meet omega-3 needs through foods instead.

Learn how to choose quality fish oil supplements.

The bottom line

Omega-3s are an important dietary component, and fish oil supplements have been tied to numerous health benefits.

Still, excessive fish oil intake can lead to side effects such as acid reflux and headaches.

Experts recommend obtaining most of your omega-3s from seafood rather than supplements. Rely primarily on whole food sources to meet recommended omega-3 intake.

If you can’t reach recommended levels through seafood alone, supplements can help you achieve adequate omega-3 status.

Read about different omega-3 supplement types and how to select the best option for your needs.